The use of semiconducting oxides as gas sensors, particularly where the oxide is used in a pressed and sintered powder form, has been common for several years. The electrical resistance of such sensors has been found to vary in a predictable manner when the sensor is operated in the presence of a particular gas, or concentration of gas, thus facilitating detection of particular gases or gas concentrations. Prior art semiconductor gas sensors however suffer from lack of stability, lack of reproducibility, and sensitivity to the relative humidity in the atmosphere. The reason for the sensitivity, and to a great extent the reason for the lack of stability and the lack of reproducibility, is believed to be associated with the fact that prior art gas sensors depend on intergranular contacts between powder grains for their sensitivity. For example, in a prior art gas sensor based on tin oxide, the gases in the atmosphere interact at the surface of the tin oxide grain, affecting the intergranular contacts and thus affecting the electrical properties of the sintered powder. Because of the sensitivity of the surface to humidity, and because of the difficulties with reproducibility of intergranular contacts, the sensors have the abovementioned problems.
The inventors have found that bismuth molybdate (a term which is hereafter used to describe an oxide where bismuth and molybdenum are cations of various atomic percentages and oxygen is the anion) can be used as a gas sensor with good sensitivity for certain gases and good reproducibility and stability, and in particular almost zero dependence of the sensor characteristics (the electrical resistivity) on the relative humidity. Bismuth molybdate gas sensors can be particularly useful for the detection of alcohol in the breath, having both substantial sensitivity in the concentration range of interest (200 ppm) and having negligible response to the humidity from the breath. There are certain other organic species that the sensor will detect, as will be described.
The inventors have also found that bismuth iron molybdate (Bi.sub.3 FeMo.sub.2 O.sub.12) can be used as a gas sensor in much the same way and with much the same properties as bismuth molybdate. The inventors have also found that the sensitivity and linearity of the bismuth iron molybdate sensor can be improved by mixing with platinum black. These compounds are part of a whole class of catalysts (molybdates) which the inventors expect to have much the same properties.
Although not wanting to be bound by any theory, the inventors believe that the insensitivity of a bismuth molybdate sensor to humidity arises because the changes in electrical resistance exhibited by the bismuth molybdate class of materials, when exposed to gases, are associated with the materials' bulk properties, rather than surface properties (intergranular contacts) as is the case with tin oxide and other such sensors. The resistance is associated with the bulk because lattice oxygen vacancies diffuse extremely rapidly through the semiconductor. This means bulk equilibrium of the catalyst stoichiometry (metal to oxide ratio) with the surrounding gas can be very rapid.
Bismuth molybdate sensors can be used in the form of a thin film of material or as a sintered powder. The thin film form is particularly useful where low sensor power and fast response time is important. In the case of bismuth molybdate which exists as several crystallographic structures the stoichiometry (ratio of bismuth to molybdenum atoms) is found to be important. A mixture of the alpha (Bi.sub.2 Mo.sub.3 O.sub.12) and gamma (Bi.sub.2 MoO.sub.6) phases of bismuth molybdate provides the optimum gas sensitivity.